The Labour leader's energy promise has been a stunning presentational and tactical success so far. Photograph: Yui Mok/PA

When "the Quad" – the government quartet composed of David Cameron, George Osborne, Nick Clegg and Danny Alexander – met on Thursday, they met alone. Officials and aides were banned from the conclave. In the words of a person extremely familiar with how they operate, this is what the coalition's leading figures do "when they want to talk dirty" without anyone else listening in.

They had hoped to be in a better place this weekend after the publication of the strongest growth figures in three years. The plan was to use that to put Labour on the defensive with a "blitzkrieg" against Eds Balls and Miliband by scorning their previous predictions about the economy. The coalition has instead found itself on the back foot and reflecting on some rocky days for the government after another week fought on living standards, Labour's preferred battlefield, and dominated by Labour's pledge to freeze energy bills.

It has been a hit, a palpable hit. Whether or not you think it adds up, Ed Miliband's promise has been a stunning presentational and tactical success so far. That's accepted even by many of those who think the actual policy is nonsense-on-a-stick. A Tory MP goes so far as to call it a stroke of "genius". One pledge that wouldn't add a penny to government spending has pumped up Labour morale and boosted their leader's self-confidence. Friends report that he thinks it has probably been the biggest contributory factor to the recent lift in his personal poll ratings. That Miliband gambit has been the dominant theme of domestic politics since September, exciting the Labour conference at which it was announced and then shadowing the Tory gathering that followed. It has thrown the coalition into bickering disarray about how to respond and put David Cameron in a corner, culminating with his crimson-faced performance at the most recent prime minister's questions. That was the fourth successive bout of mouth-to-mouth combat between the Tory and Labour leaders which was dominated by energy bills. I'm trying to remember the last time that one topic monopolised PMQs for four sessions in a row. You'd probably have to go back to the time of the Iraq war a decade ago. If one of the tests of a politician is the ability "to make the weather", this pledge has certainly passed it.

There is a huge irony here. I can reveal that for a while it was touch and go whether Labour would actually unveil the promise. Some of Mr Miliband's closest advisers and senior colleagues were highly sceptical about the wisdom of announcing it at the party conference or even of pledging it at all. Some thought they already had enough policy to reveal in Brighton without needing that as well. Others worried that the energy bill pledge would be dismissed as gimmicky and short-termist. Then there were those anxious that it would be taken as an "anti-business" signal.

"Everyone wanted the tyres kicked," says one very senior member of the shadow cabinet. The Labour leader went ahead because he desired a clear and populist "retail offer" to the voters. He was obviously influenced by the private polling which told him that energy bills were the starkest example of the squeeze on their living standards in the minds of many voters, and a freeze was wildly popular when it was tested on focus groups. To the average household, a 20-month cap would probably be worth something over £100, not a huge sum of money relatively speaking. Yet the pledge's impact has been larger and longer lasting than anyone anticipated, either on the Labour side or in government.

It has cut through partly because the proposal is simples for the media to grasp and turn into a headline. It is partly because Labour has excelled at something at which it is often poor: it has kept ramming home its message, punching the bruise. Then there has been a bit of help from the energy companies. When public tolerance for inflation-busting price rises was already exhausted, they played into the hands of Mr Miliband by announcing more whopping increases. He owes the energy bosses some peerages for their services to the Labour party.

Just when interest might have started to fade came the surprise intervention by Sir John Major. "We were not expecting that," says one of Mr Cameron's aides. "Are we going to pretend it was helpful? No." The former Tory prime minister unleashed what you might call a bolt from the grey. He said that some families will have to choose whether to eat or heat their homes this winter. That intervention was more salient because Sir John carefully rations his public appearances and has previously displayed no inclination to cause trouble for David Cameron. Sir John proposed a windfall tax on the energy companies. While a different answer to that suggested by Mr Miliband, the intervention set up Mr Cameron for his fall at PMQs by keeping the debate on Labour's turf and undercutting the prime minister's favoured line of attack on his opponent. It will be hard to persist with the claim that the Labour leader is some kind of neo-Marxist unless the Tories are also going to call their last leader to win an election a crypto-Leninist.

The final – and probably most important – reason that this one Labour pledge has been so disruptive to the coalition is because the government has so obviously thrashed around trying to muster a coherent response. It has exposed one of the most significant fault lines between blue and yellow. The Lib Dems' instinct would be to press hard on the energy companies, but they have struggled to bring along their Tory partners. The Conservatives are "congenitally averse to taking on corporate interests" in the words of one senior Lib Dem minister. Just half an hour before PMQs, David Cameron informed Nick Clegg that he was going to tell MPs that energy bills could be brought down by "rolling back" the green component. He did this in defiance of earlier private warnings from the Lib Dem leader that this was a "bloody stupid" idea that would "keep the ball in Miliband's court". It would make a nonsense of David Cameron's previously proclaimed desire to lead "the greenest government ever". And, so Mr Clegg warned Mr Cameron beforehand, the Lib Dems were simply not going to agree. As a result of this split within the coalition, they have got on the wrong side of the cost of living argument. Ultimately, I expect the coalition parties will compromise and find a response. Both know they need one. "Smoke signals" from the Quad meeting suggested to other members of the government "that a deal can be done". Ministers currently expect it to emerge in George Osborne's financial statement later in the year. But unless they come up with something terribly clever it will look as if they are playing catch-up and playing it very slowly.

So Labour can be pleased with itself. Yet it should beware getting too intoxicated with this one victory. Which party looks best on living standards will be a key feature of the political battle between now and the next election, but it will not be the sole component. Labour will also need a plausible account of how Britain is going to prosper in the future. The party has some ideas about that, but they have yet to find a way to present them that cuts through. Crucially, Labour still has a large deficit on the question of overall economic competence. Tories acknowledge that the energy price freeze is highly popular, but claim that their polling shows that many people are sceptical that it would actually be delivered. A general mood of cynicism towards any politician bearing gifts is compounded by voter doubts about Labour as a steward of the national finances. Says one Tory: "People just don't believe in free lunches."

Cabinet ministers accept that Labour has done well to "change the channels", as one senior government member puts it. Labour is best placed while the focus is concentrated on living standards, the issue on which it is ahead, and attention is away from economic competence, where the Tories have the advantage. But the Conservatives will make a huge effort to switch the focus back to competence, where they were ahead for most of the time even when the economy was in recession. They have already told us how they will attack Labour: "Don't give the keys back to the guys who crashed the car." As one Tory puts it: "We will be repeating that message relentlessly, monotonously, monomanically from now until polling day." The Lib Dems also have an incentive to get the argument back on to this territory if Mr Clegg is to fulfil his goal of getting some credit for economic recovery for his party.

There are some Labour people who worry that, popular though it is, the energy price pledge is working against them on economic credibility by crowding out anything else the party has to say and implanting the notion that they think that there are easy fixes to every hard problem. Labour needs to be wary that voters don't conclude that the energy price cap is its only economic policy.

With this stroke, Ed Miliband has scored a hole in one. But that won't serve him well in the end if people think that he is a golfer with only one club.